1. Field of the Invention
This invention relates to a liquid-crystal display architecture.
2. Discussion of Prior Art
The majority of liquid-crystal displays (LCDs) currently manufactured are twist-cell displays. There are two generic types, Twisted Nematic (TN) and Super-Twisted Nematic (STN). Both involve the sandwiching of a thin (several microns) layer of nematic liquid-crystal (LC) between two flat glass plates. Both glass plates are coated on the inside with a transparent but conducting electrode layer, on top of which is deposited an alignment layer (commonly rubbed polyamide). In the locality of the alignment layer the LC molecules (which can be thought of as rod-like) tend to lie flat and point in the rubbing direction. In the TN device the rubbing directions on the top and bottom plates are perpendicular to each other and impose a 90 degree twist in the molecular LC layer. STN devices induce a twist usually greater than 180 degrees by the use of a chiral additive and the correct positioning of the rubbing directions. TN and STN are considered to represent two completely separate classes of device.
The conductive layers enable an electric field to be applied across the LC material. Such an electric field can cause the LC molecules to align themselves parallel to the field (perpendicular to the glass), destroying the twisted structure.
When in the twisted state the LC molecules tend to rotate the polarisation of incident light. This rotation no longer takes place when the application of a large enough electric field destroys the twist of the LC material. By placing twist cell devices between two linear polarisers, such polarisation modulation can be converted to a modulation of intensity.
A display comprises many such cells, or pixels, in an addressable array. If each pixel is connected directly to a dedicated drive transistor (direct drive/active matrix), the optical effect of TN devices outperforms that of STN devices. However, when arranged in a matrix of rows and columns, with single drivers at the end of each row and column (passive matrix), the STN configuration vastly outperforms the TN--it is well known that TN devices can only maintain acceptable contrast if passive-matrix multiplexed into fewer than thirty or so rows, whereas an STN display may maintain similar contrast with around 200 multiplexed rows.
STN devices have the added complication of chromaticity--they operate in either a black-and-yellow or white-and-blue mode. In order to make a true colour STN display, complicated techniques are necessary to compensate for this colour change.
Liquid-crystal displays can be either reflective or emmissive. Established twist cell LCD architectures of the emissive type use a large-area whit backlight to flood a LC shuttering layer with light. A picture can be seen by the viewer from all the angles at which light passes through the shutters. However, the quality of the picture seen tends to degrade rapidly as the viewing direction moves from the normal, an effect primarily due to the angular dependence of the LC electro-optic response.
However, it is a known phenomenon of TN LCDs that as the number of passive-matrix multiplexed lines is increased beyond the normally acceptable limits of contrast, brightness, and viewing angle there remains a small cone of angles lying off axis around which a picture can still be seen (though not visible from any other direction).
Goscianski (1977) investigated this sharpening of the TN transfer function (i.e. of the electro-optic effect) with polariser position for on-axis illumination, and developed a highly multiplexed transmissive TN display using collimated light. He pointed out that this configuration was suitable for projection displays, or with a diffuser could be used with a viewing angle of about 30.degree..
At about the same time, Kahn and Birecki investigated the sharpening of the transfer function with the direction of illumination, using a standard TN cell between crossed polarisers illuminated with collimated light. They identified the optimal angular range and that this could be achieved by tilting the cell.
Such displays however have not been practical, since the low range of viewing angles of these prior art devices, even with diffusers, has been too small except for use in projection displays. This situation illustrates the general point that nematic (and other liquid crystal) cells tend to have contrast ratios and multiplexability that depend strongly on the angle at which light traverses the liquid crystal layers. There is therefore a need for a display using a TN or other nematic cell which is operated and aligned optimally but with a wide viewing angle.